When injected into an individual with diverse adjuvants, protein antigens usually stimulate the production of high-affinity IgG antibodies, indicating that they activate CD4 T helper cells, as well as B cells. These procedures generally fail, however, to elicit effective CD8 T cell responses. The reason, according to current views, is that the short peptides needed, in association with MHC class I molecules, to stimulate CD8 T cells arise from proteolytic cleavage of cytosolic proteins. Since injected protein antigens are generally unable to cross cellular lipid membranes, they fail to gain entry to the proper cytosolic “MHC class I processing pathway” and are thus unable to stimulate the production of CD8 T cells. Although there is evidence for alternative cellular pathways for processing some exogenous proteins to form peptide MHC class I complexes (Sigal, L. J., et al., Nature, 398:77-80 (1999) and Gromme, M., et al., Proc. Natl. Acad. Sci. USA, 96:10326-10331 (1999)) it remains generally true that protein antigens normally fail to stimulate significant CD8 CTL responses (Rock, K., Today, 17:131-137 (1996)).
There is now substantial evidence that heat shock proteins (hsps) isolated from tumors can be used as adjuvant-free anti-tumor vaccines in animals; hsp70 and the distantly related chaperones gp96 and calreticulin share this immunostimulatory activity (Udono, H. and Srivastava, P. K., J. Exp. Med., 178:1391-1396 (1993); Udono, H., et al., Proc. Natl. Acad. Sci. USA, 91:3077-3081 (1994); Suto, R. and Srivastava, P. K., Science, 269:1585-1588 (1995); Blanchere, N. E., et al., J. Exp. Med., 186:1315-1322 (1997); Tamura, Y., et al., Science, 278:117-120 (1997) and Nair, S., et al., J. Immunol., 162:6426-6432 (1999)). The fusion of large polypeptides (80-110 amino acids in length) to mycobacterial hsp70 (TBhsp70) creates potent immunogens that can elicit MHC class I-restricted, CD8+ cytotoxic T cell responses sufficient to mediate rejection of tumors expressing the fusion partner (Suzue, K., et al., Proc. Natl. Acad. Sci. USA, 94:13146-13151 (1997)).
The means by which soluble hsp70 fusion proteins stimulate CD8 cytotoxic T cell (CTL) responses are unknown. Among the possible mechanisms are: 1) strong hsp-specific CD4+ helper cell responses that enhance what might otherwise be a minimal response to the soluble proteins (Barrios, C., et al, Eur. J. Immunol., 22:1365-1372 (1992); Suzue, K. and Young, R. A., J. Immunol., 156:873-876 (1996); Horwitz, M. S., et al., Nature Med., 4:781-785 (1998) and Könen-Waisman, S., et al, J. Infect. Dis., 179:403-413 (1999)); and 2) chaperone function of hsps delivers the fusion protein to intracellular compartments of antigen-presenting cells for processing into short peptides and loading onto MHC class I (Young, R. A., Ann. Rev. Immunol., 8:401-420 (1990) and Schild, H., et al., Curr. Opinion Imm., 11:109-113 (1999)). An understanding of the ability of hsp70 to stimulate CD8− CTL responses is needed to provide for more effective immunological prophylaxis and therapy for cancer and infectious diseases caused by intracellular pathogens.